1. Field of the Invention
The invention relates to a clamping screw for fastening an electrical conductor in a metallic terminal which is designed as a tubular piece and which in its wall has at least one through hole provided with a thread and intended for receiving the clamping screw, wherein a ball is mounted in a recess at that end of the clamping screw which serves to bear against the electrical conductor (DE 1 902 067 U).
2. Description of Related Art
The contact pressure with which a clamping screw in the mounted state presses against an electrical conductor should be as large as possible and be maintained permanently so that a stable electrical connection between electrical conductors of cables, in particular of power cables, can be ensured throughout the required service life. Owing to the friction between the clamping screw and the thread of the through hole of the terminal, and owing to the friction between the end face of the clamping screw and the surface of the conductor when tightening said clamping screw, an increased force is required to turn the clamping screw. As a result, the force with which the clamping screw presses onto the conductor may be too low to achieve a permanently sufficient contact stability.
Added to this is the fact that, in the case of multi-wire conductors, the individual wires thereof rub against one another when compressed as a result of the clamping screw being tightened. This leads to a lower pressure being produced inside the conductor than on the surface thereof. That can lead to problems when connecting multi-wire conductors having a large cross section or in the case of segmental conductors, so-called “Milliken conductors”. If the contact between the individual wires inside the conductor is insufficient, above-average heating occurs. In the case of conductors made of aluminium, the heating additionally leads to an increased flow of the material and consequently to a further reduction in the contact pressure after only a short time. The electrical connection between the clamping screw and conductor or between the conductor and terminal is then unusable relatively quickly.
DE 102 30 502 A1 discloses a clamping screw at whose end there is situated a clamping piece portion which is designed as a disc and which is set off all around with respect to the clamping screw by a lateral indentation to form a predetermined breaking point. During the tightening of the clamping screw, the force on its end face grows and the disk is separated from the clamping screw at the predetermined breaking point. The clamping screw can subsequently rotate relative to the disc which is then pressed, only without rotating itself, onto the conductor in the axial direction of the clamping screw. Prior to the separation, the disc is rotated together with the clamping screw and thereby moved or rotated on the conductor. This can lead to undefined damage to the conductor. Moreover, particularly in the case of multi-wire conductors having a large cross section, the relatively large-area disc does not allow a high contact pressure inside the conductor.
DE 1 902 067 U, mentioned in the beginning, describes a clamping screw designated as a pressure screw wherein a ball constituting a pressure piece is held with a limited degree of axial mobility in a recess in the end of the screw shank. The recess is of such size that the ball can be displaced therein in the axial direction of the screw shank to such an extent that it completely disappears within the profile thereof when the clamping screw is tightened for fastening to an electrical conductor. Here, said ball is moved counter to the action of a compression spring mounted in the screw shank. Therefore, when tightening the clamping screw, at first only the end face or shoulder thereof acts on the conductor. It is only when signs of fatigue appear on the conductor that the ball is pressed against the latter by the compression spring. In the case of this known clamping screw, too, the end face of the clamping screw is thus rotated further in the circumferential direction after coming into contact with the conductor, which means that damage to the conductor cannot be ruled out here either.